Fuse mechanism



March 22, 1932. T. E. MURRAY, JR

FUSE MECHANISM Filed Aug. 4, 192e XSQMMM Patented Mar. 22, 1932 UNITEDSTATES PATENT OFFICE THOMAS E. MURRAY, JR., OF BROOK'YN, NEW YORK,ASSIGNOB. T0 METROPOIJI'IAIVQv DEVICE CORPORATION, A 'CORPORATION OF NEWYORK FUSE Mn'cIIANIsM Application led August 4, 1922. Serial No.297,527.

improvements in such circuits and fuses.

The accompanying drawings illustrate an embodiment of the invention.

Fig. 1 is a side elevation, partly in section of the invention appliedto a knife switch;

Fig. 2 is a vertical section of one of the fuses. v

Referring to Fig. 1, the incoming bus bar 1 of the circuit is mounted onthe barrier or other support 2. On the opposite side of the barrier is aswitch blade 3 pivotedon a support 4 which is connected through thebarrier tothe bus 1. The switch blade may be actuated by means of an arm5 connected by an insulated link 6 with the core 7 Of a quick-actingsolenoid 8.

The outgoing bus 9 also mounted on the rear of the barrier carries fixedblade 10 in line with the opening 11 in the barrier registering with thelateral projection 12 on the switch blade.V Normally the projection 12engages the contact 10 and the normal current flows from one to theother ,bus bar throu h the switch blade in the manner indicated y thefull line arrows.

The switch blade 3 has an extension 13 carrying a set of end contacts 14adapted to engage respectively contacts 17. These are separate contactseach Of which is separately mounted on a bracket`20 supported froml aninsulating bracket 21. Between the bus bar 9 and the several brackets 20there are inserted a number of fuses corresponding to the number ofswitch blades 17 (which may be any `desired number).

The fuses are preferably ofl a type covered in certain priorapplications of Murray; for example Serial No. 252,439 iled February 7,1928. t

Fig. 2 shows one of the fusesin section. An open ended tubular case 22carries a pair of plungers 23 of insulating imaterial* adapted to slidefreely in the case. Thefuse strip 24 has a blowing point 25 locatedbetween the plungers at points adjacent thereto. The fuse strip isimbedded and fastened in the plungers. The gas generated b the blowingof the fuse is, therefore, con ed and the plungers with their engagedportions of the fuse strip are blown quickly away from the blowing pointand out of the open ends of the casing or to such a distance that thecircuit cannot be re-established by the passage of the current throughthe gas generated.

.The material and dimensions of the fuse strip and the spacing betweenthe plungers are so calculated that the portion of the strip between theplungers will, on gasification, be suiiicient to give the desired quickand wide separation of theplungers with their ends of the fuse strip.The amount of the fusible metal which will be fused or gasied when theblow out occurs is determined by the spacing'between the plungers, andis limited b the plungers. The plungers embed the fuse strip so closelythat the arc cannot penetrate to a substantial extent, so that thespacing of the plungers determines the quantity of metal v which will be'gasified The leads 26 and 27 to and from the fuse strip are located onopposite sides of a horizontal barrier 28 projecting outward from theback of the main barrier 2. Bythis arrangement we have a repulsioneffect between the two conductors 26 and 27 on the opposite lsides ofthe insulating barrier, which also tends to throw the ends of thel fusestrip apart when the reduced portion thereof burns through. The singlebarrier 28 is used for the entire group of fuses, the casings of thelatn ter being fixed in the outer end of the barrier.

Now, when it is desired to break the circuit under a load between thebusses 1 and 9, the switch blade 3 isswung outward. When the contact 12leaves the Contact 10, the main connectionbetween the two busses isbroken. But there is still a path for the current as indicated by thedotted line arrows through the extension 13 of the switch blade and thecontacts 14 and 17 and through the lfuse to the outgoing bus 9.

Fig. 1 shows this, which is an intermediate position of the parts. rIheoutgoing movement of the switch blade will be continued beyond theposition shown. But when this E position occurs the low of the fullcurrent through the fuse blows the latter and completely breaks thecircuit between the busses.

The further movement of the switch blade carries the contact 14 clear ofcontact 17.

Two or more fuses may be used in parallel, but ordinarily one fuse inthe circuit will serve the desired purpose. When it is blown a new fusemay be substituted by the use of any ordinary fuse replacement device.

The ,fuse thus serves as a protection for `a-n instant during which theswitch is being opened. The current having a lpath through the fuse atthis instant there is no arcing across the main switch contacts and noburning or pitting of the latter;A

Y When the switch is opened and the fuse blows, the plungers or pistonsare separated rapidly. But for a time the .electrical circuit is carriedbetween them by the metallic vapor produced by the blowing. -It isimportant to open the circuit as quickly as possible.

I have found that this can be done by cutting the size of the tube andplungers to very small proportions. In fact, I h ave used a fuse havinga tube 2% inches long and 3/8ths of an inch in diameter with acombustion chamber (thespacebetweentheplungers) of .027v to .497 cubicinches and such a fuse has operated satisfactorily under short circuitconditions on 2500 volts, 60 cycles, using 100 amperes. It opened thecircuit in less than l@ of 'a cycle. This represents a time of less than1/240th of a second, a speed previously unheard of 1n electricalpractice. I

The intensity of a short circuit 1s proportioned to the length of timetaken to open or break it. As the time is lengthened, the shortl isliable to build up enormous values. For example, a 100 ampere fuse on240 volts might draw a load from the main line. or generator of 100times its normalcapaclty, that is 10,000 amperes. Its eiiiciency dependson the speed with which it can clear the circuit.

In the eort to find the smallest fuses available for practicaldimensions of the fuse ribbons, I have determined that the lengthof thetube should be at least `about eight times its internal diameter.' Forexample, a 1/2 inch tube for 2600 volts should be four to iive incheslong, for 6,600 volts, six to eight inches and for v11,000 volts, twelveto fourteen inches. If the inside'diameter is changed, 6@ the lengthshould be changed in proportion.

If the above fuses have a diameter of one inch, their lengths should bedouble the above figures.,` These figures are approximate.

But they represent fairly the best propor- 55 tions available with theordinary fuse ribbons to secure the quickest complete clearing of acircuit VOther metals than the ordinary fuse metal which isapproximately a composition of solder, would make it possible to reducestill further the size of the fuse and the quickness of its operation.Silver, for example, having a higher electrical conductivity, wouldcotton are xed on the top of the lower plunger.

I propose to use, thereforean extremely small fuse on high tension linescarrying normally a number of amperes greatly in excess of the capacityof the fuse, the latter being of the minimum size which will carry thecurrent long enough to permit the completev withdrawal ofthe switchcontacts without arcing.

I Ypropose also to provide a certainresistance, indicated at 30, Fig. l,in the line of the fuse so as to retard the flow of the full currentthrough this line and to prevent the blowing of the fuse until theswitch contacts are completely separated; that is separated to such adistance that'there is no danger of an arc across them when the fusecircuit is broken. This resistance should bev enough to overcome the E.M. F. drop across the main contacts. I have used in such circuits thefollowing fuse capacities.

Current in main circuit Size of fuse.

500 to 1000 amperes 25 ampere fuse 200 to 500 amperes 20 ampere fuse 100to 200 amperes 15 ampere fuse 75 to 100 amperes 10 ampere fuse 25 to 75A amperes; 5 ampere fuse A resistance of ten ohms in the fuse circuithas given very good results. This allows about one'ampere or less toflow through the lise circuit during regular operating condilons.

No claim is made herein to the fuse separately, the same being claimedin my application No. 320,7 95, filed November 21, 1928.

Various modications in detail may be made by those skilled in the artwithout departing from the invention as defined in the following claims.

1. An electrlc circuit-breaker comprising a switch, a fuse adaptedtocarry a portion of 2. An electric circuit-breaker comprising a switch, acasing, a fusible element therein, a plunger on said fusible elementadjacent its blowing point confining the gas generated by blowing thefuse and causing it to expel the plunger, said fusible element adaptedto carry a portion of the current under normal operating conditions andduring the opening of the switch and to break the circuit thereafter,and means for preventing excessive current through the fusible elementunder normal operating conditions.

3. An electric circuit-breaker comprising a fuse adapted to carry aportion of the current under normal operating conditions and during theopening of the switch and to break the circuit thereafter, said fusebeing of the minimum size which will carry the current long enough topermit the complete withdrawal of the switch contacts without arcing,and a resistance in line with the fuse to prevent excessive currentthrough it under normal operating conditions.

4. An electric circuit-breaker comprising a fuse adapted to carry aportion of the current under normal operating conditions and during theopening of the switch and to break the circuit thereafter, said fusecomprising a casing, a fusible element therein, a plunger on saidfusible element adjacent to its blowing point conlining the gasgenerated by blowing the fuse and causing it to expel the plunger, saidfuse being of the minimum size which will carry the current long enoughto permit the complete withdrawal of the switch contacts without arcing,and a resistance in line with the fuse to prevent excessive currentthrough it under normal operating conditions.

5. An electric circuit-breaker comprising a switch, a fuse adapted tocarry a portion of the current under normal operating conditions andduring the opening of the switch. and to break the circuit thereafterand a re- I sistance in line with the fuse to prevent excessive currentthrough it under normal operating conditions, said resistance beingsuflicient to retard the flow of the full current through the fuse andto prevent the blowing thereof until the switch contacts are completelyseparated.

6. An electric circuit-breaker comprising a fuse adapted to carry aportion of the current under normal operating conditions and during theopenin of the switch and to break the circuit therea er, said fuse beingof the minimum size which will carry the current long enough to permitthe complete withdrawal of the switch contacts without arcing, and aresistance in line with the fuse to pretacts without arcing, and aresistance in line with the fuse to prevent excessive current through itunder normal operating conditions, said resistance being suflicient toretard the iow of the full current through the fuse until the switchcontacts are completely separated.

ln witness whereof, l have hereunto signed my name.

THOMAS E. MURRAY, JR.

vent excessive current through it under normal operating conditions,said resistance being suliicient to retard the iow of the full cur rentthrough the fuse until the switch contacts are completely separated.

